As shown in FIG. 4, conventionally suggested as one of holographic recording methods is a holographic multiplex recording method based on angle multiplexing, in which the incident angle of a reference beam 6 is modulated while the incident angle of an object beam 4 is kept constant with respect to a recording layer 2A in a holographic recording medium 2 (e.g., see Japanese Patent Laid-Open Publication No. 2003-178460).
Note that since such angle multiplex recording allows a number of multiplexed holograms to be provided in the same area of a recording material, the amount of exposure for recording needs to be increased step by step as the multiplex recording proceeds. Thus, “scheduling of recording” is carried out to control the amount of exposure for recording according to the history of recording on the recording material or the residual dynamic range.
This “scheduling of recording” will now be explained in more detail with reference to FIG. 5. Note that FIG. 5 shows the relationship among the amount of exposure, the refractive index modulation degree, and the recording sensitivity for the holographic recording medium 2, in which the maximum refractive index modulation degree for a photosensitive material of the holographic recording medium 2 is n1, and both the refractive index modulation degree and the recording sensitivity are normalized to have a maximum value of one.
For example, to record N data pages by angle multiplex recording, the maximum refractive index modulation degree n1 is divided into N equal parts, and the refractive index modulation degree per data page n1/N is kept constant. However, since a photosensitive component in the photosensitive material decreases as the multiplex recording proceeds causing degradation in photosensitivity (recording sensitivity) to a laser beam, the refractive index modulation degree and the exposure time t exhibit a nonlinear relationship. Accordingly, in order to keep the refractive index modulation degree per data page n1/N constant, the exposure time t needs to be varied for each data page to increase the amount of exposure E(t) step by step.
As described above, the time t of exposure to a laser beam had to be varied in the conventional recording scheduling. Accordingly, this causes control to be readily complicated and the time required for recording to vary for each data page, resulting in inconsistency in recording data rate.
Furthermore, there is a problem with the holographic recording that microscopic vibrations or scintillations during recording cause blurring (nonuniformity in recording) to occur in interference fringes. In addition, there is a problem that different exposure times t for each data page cause such blurring to have inconsistent effects thereby impeding proper data readout.